The invention relates to scintillation cameras, and more particularly relates to signal processing techniques which may be used in such cameras. In its most immediate sense, the invention relates to signal processing technology which is used to unpile so-called "pile-up" events.
A scintillation camera converts scintillation events in a scintillation crystal into electrical signals, which are later processed to produce a planar or tomographic image of a region of interest in a patient's body. Such signals are not equally useful; some contain a favorable ratio of information to statistical noise, while others contain an unfavorable ratio. An accurate image cannot be based on erroneous information, and it is therefore necessary to test the signals for the likelihood that they contain useful information, rejecting ("dumping") those with lesser information content and accepting and using those with more information content.
One such test (herein referred to as "energy-validation") is for energy content. In a conventional scintillation camera, for example, the electrical pulse from a scintillation event is integrated to derive a measure of the incident energy of the photon which generated the event. Then, this derived measure is compared with a range, or "window", of acceptable energies. If the derived energy of the event falls within the window, the event is considered to contain useful information, and the pulse is considered "valid" and used in subsequent camera systems. If the derived energy of the event falls outside the window, the event is considered to contain large quantities of noise (it may, e.g., be a "scattered" event which is deflected in the body), and the "invalid" pulse is dumped.
However, there are circumstances in which such a test cannot produce meaningful results. One such circumstance, commonly referred to as a "pulse pile-up" or "pile-up", takes place when two scintillation events follow each other so quickly that they overlap. When such an overlap occurs, the two resulting electrical pulses become superposed. In this situation, the integrated superposed pulse and its coincidence with the energy window have no meaning, because, in the general case, there is no a priori way to determine, from the integrated superposed pulse, whether the superposed pulse results from two valid pulses, two invalid pulses, or one invalid pulse and one valid one. This in turn comes about because the degree of overlap between the two piled-up events is not known in advance.
This problem has long been recognized, and has been dealt with in various ways. One approach is to identify the existence of a pulse pile-up and to dump all pulses which contribute to it. While this approach prevents invalid events from being further utilized, it discards data which may well represent valid events. This effectively reduces the rate at which meaningful data are acquired and consequently makes it necessary to prolong a patient study. This decreases patient throughput through the camera and increases costs, and is consequently undesirable.
Another approach is to use the occasion of a pulse pile-up to simulate the tail of a preceding pulse. This simulated tail can be used as a correction to the preceding pulse and can be used in subsequent processing in the stead of the actual pulse tail which it simulates. This approach suffers from the disadvantages that the simulation is not necessarily accurate over a wide range of pile-up conditions and that it is undesirable to process simulated data instead of actual data.
One object of the invention is to permit a pulse pile-up to be unpiled with a high degree of accuracy under a relatively wide range of pile-up conditions.
Another object of the invention is to reduce, as much as possible, the need to discard piled-up pulses which have been generated by valid scintillation events.
A further object of the invention is to use actual data, rather than simulated data, in the construction of planar and tomographic images, even in the event of pulse pile-ups.
The invention proceeds from a realization that the use of conventional pile-up detection circuitry makes it difficult to unpile pulse pile-ups. This is because conventional circuitry is of the analog type. As a result, the shapes of pulses and pulse pile-ups are not preserved in the camera head electronics. An accurate determination of the existence of a pulse pile-up and an accurate unpiling of the pile-up requires accurate analysis of wave shapes. Consequently, such analysis is more difficult when analog circuitry is used to carry the analysis out.
In accordance with the invention, pulses generated by scintillation events are sampled at a high frequency. This preserves the shape of the pulses. In further accordance with the invention, the samples are summed together, thereby effectively integrating the pulse, but on a progressive basis and not over a fixed predetermined time constant, and the resulting integral is stored. If the next pulse is piled-up upon a previous pulse, the next pulse can usually be unpiled and energy-validated using the stored information about the previous pulse. Should the next pulse, after energy-validation, be found valid, the stored information from the first pulse is replaced by information from the next (and now validated) pulse. If the next pulse cannot be unpiled, or if the next pulse is not piled-up on its predecessor, the stored information is purged.
In further accordance with the invention, there is provided an algorithm which is particularly suitable to pipeline-type operation, such as exists in a scintillation camera head. When pile-ups take place, it is normally possible to unpile the first of the piled-up pulses and to store information about it while the second piled-up pulse is being unpiled (with the aid of the stored information). After this has taken place, the originally-stored information is purged and replaced by information about the second piled-up pulse, for use if necessary in unpiling a third piled-up pulse, and so forth. Only where two pulses are so closely simultaneous that they cannot be unpiled are those pulses (and any subsequent pulses which may be piled-up upon them) dumped.
In yet further accordance with the invention, a pulse under investigation is energy-validated before it has died out. This is accomplished by extrapolating the remaining portion of the pulse and determining whether the time integral of the pulse--including the extrapolated portion--falls within the desired energy window. Where the extrapolation takes place at the beginning of a pile-up of a second pulse upon a first pulse, the time integral of the extrapolated portion of the first pulse can be computed even before the first pulse has died out. The resultant time integral can then be subtracted from the time integral which commences with the beginning of the second pulse and ends at the end of the second pulse (or with the beginning of a third piled-up pulse, if this occurs). The difference can then be compared with the energy window to energy-validate the second pulse.
With the exception of energy-validation, the above-described aspects of the invention are equally applicable to the channels of a scintillation camera which process the X and Y coordinate data, and are not restricted to the Z (energy) channel. The invention is not restricted to use in the Z channel of a scintillation camera.